The regulation of osteoclastic bone resorption is a precise, complex process involving a number of mediators and cell types. Osteoblasts are among the major cells responsible for both recruitment and activation of osteoclasts. They act in a variety of ways to effect osteoclast development. This proposal uses 3 different osteopetrotic mutation in the rat, with known reductions in bone resorption and aberrations in osteoblast gene expression, bone matrix components and bone cell proliferation, to explore the regulation of bone resorption by osteoblasts and bone matrix constituents. Our specific aims are to determine the relationships of the osteoblast and osteoclast abnormalities in these 3 mutations, to characterize and quantitate protein synthesis and its regulation in mutant osteoblasts, to determine the ability of mutant osteoblasts and matrix products to recruit and direct differentiation and function of normal osteoclasts and to evaluate the role of the cytokine M-CSF in osteoblast and osteoclast function. To this end we will use cure of the disease in mutants and induction of the disease in normal littermates as mechanisms to perturb and follow osteoblast and osteoclast function by molecular techniques; mRNA levels of osteoblast-specific genes, +/- cDNA library screening and subtractive hybridization to detect unique genes of the osteopetrotic phenotype, and PCR amplification to dissect aberrant genes by nucleotide analyses. Osteoblast gene expression and its hormonal regulation will be compared in cultures of mutant and normal osteoblasts and metatarsal cultures and bone slices will be used to analyze the ability of mutant bone matrix and osteoblasts to direct osteoclast differentiation and function, respectively. Finally, the ability of M-CSF gene expression in this mutation, and to study the role of this cytokine in skeletal development by in situ hybridization. Future studies will be directed toward gene therapy for this mutation. These studies are expected to provide insights into the cellular and molecular bases for the regulation of bone resorption and have application in situations where selective regional or general reductions or accelerations of bone resorption are desirable.